PROJECT SUMMARY (Abstract) In addition to genetics and environment, interindividual variation in epigenetic regulation may determine risk of obesity. Exceptional genomic loci called metastable epialleles offer unprecedented opportunities to test this. Metastable epialleles ? essentially epigenetic polymorphisms ? exhibit interindividual variation in DNA methylation that is neither tissue-specific nor genetically mediated. Metastable epialleles were first discovered in mice when dramatic phenotypic variation was observed amongst inbred mice. Over the last 10 years we pioneered the discovery and characterization of human metastable epialleles. We have shown that DNA methylation profiling of multiple tissues in multiple individuals is an effective approach to identifying regions that exhibit systemic interindividual variation in DNA methylation, a hallmark of metastable epialleles. Now that we know epigenetic metastability is common to mice and humans, it is essential to identify and explore the full range of MEs in the mouse. We therefore propose to pursue the following Specific Aims: Aim 1 - Perform an unbiased screen for mouse metastable epialleles. Aim 2 - Test for effects of maternal dietary methyl donor supplementation on offspring DNA methylation at metastable epialleles. Aim 3: Use comparative genomics to identify sequence determinants of epigenetic metastability. Successful completion of these Aims will provide the foundation for development of mouse (and potentially other mammalian) models which will enable controlled experiments to help understand how interindividual variation in DNA methylation affects risk of disease in humans.